What is SMTP Relay and How Does It Work?

When you send an email, it does not always go straight from your email app to the recipient’s inbox. In many cases, the message passes through one or more mail servers before it reaches its final destination. This process of moving an email from one mail server to another is called SMTP relay.

SMTP relay is an important part of email delivery. Businesses use it to send everyday emails such as order confirmations, password resets, alerts, newsletters, and other automated messages. Instead of handling every email delivery step on their own, businesses can use an SMTP relay server or a third-party SMTP relay service to help send their messages.

But what exactly is SMTP relay? How does it move an email from the sender to the recipient? And what is the difference between an SMTP server and an SMTP email relay?

In this guide, we’ll explain what is SMTP relay, how SMTP relay works, the different types of SMTP relays, and why businesses use SMTP email relay services.

What is SMTP Relay?

SMTP relay is the process of transferring an email from the sender’s mail server to the recipient’s mail server using the Simple Mail Transfer Protocol (SMTP).

Think of it like a delivery network.

When you send an email, your email doesn’t usually travel directly from your device to the recipient’s inbox. Instead, SMTP servers help move the message between different systems until it reaches the server responsible for the recipient’s mailbox.

For example, when you send an email from [email protected] to [email protected], the process may look like this:

Your email client → Your SMTP server → Recipient’s mail server → Recipient’s inbox

The SMTP server responsible for passing the message along is performing an SMTP relay. An SMTP relay can also be used by applications, websites, and services that need to send emails without operating their own mail infrastructure.

How Does SMTP Relay Work?

So, what actually happens when you click ‘Send’ on an email? The message does not simply jump from your email app to the recipient’s inbox. It usually passes through one or more mail servers, and each server has a specific job in getting the message to the right place. SMTP handles this communication and helps move the email from the sender’s system to the recipient’s mail server.

This is where SMTP relay comes into the picture. A relay helps pass the message from one mail system to another until it reaches the server that handles email for the recipient’s domain. Depending on how the sender’s email system is set up, there may be one relay involved or several servers along the way.

Here is a simple step-by-step look at how SMTP relay works:

Step 1: The Sender Creates an Email

Everything starts when you write an email in Gmail, Outlook, Apple Mail, or another email application. You enter the recipient’s email address, add a subject, write your message, and attach any files you want to send. When you click ‘Send,’ your email application needs somewhere to hand the message over for delivery. This is usually an outgoing SMTP server provided by your email provider or organization.

The email also contains several pieces of information that mail servers need to process it, such as:

• Sender address: This tells the mail system who the email claims to be from.
• Recipient address: This tells the mail system where the message needs to go.
• Subject and message: This is the actual content of the email.
• Attachments: Any files included with the message are sent along with it.
• Email headers: These contain additional information that helps mail servers handle and trace the message.

Once everything is ready, your email application connects to the SMTP server and hands over the message. From there, the mail server takes over the delivery process.

Step 2: The SMTP Server Receives the Email

The SMTP server does not simply accept every email request that comes its way. It first checks whether the sender is allowed to use the server to send messages. For example, a company’s SMTP server may only accept emails from its employees, approved applications, or other authorized systems.

The server may ask the sender to prove their identity using a username and password, an API key, or another authentication method. These checks are important because without them, attackers could use the server to send large amounts of spam or phishing emails.

Once the server accepts the sending request, it looks at the recipient’s email address. More specifically, it looks at the domain after the @ symbol. If the recipient is [email protected], for example, the server needs to figure out which mail server handles email for example.com.

Step 3: The SMTP Server Finds the Recipient’s Mail Server

Knowing the recipient’s email address is not enough to deliver the message. The sending server also needs to know which server is responsible for receiving email for that domain. To find this out, it uses the Domain Name System (DNS) and looks up the domain’s MX records.

An MX record tells sending mail servers where they should deliver emails for a particular domain. So, when someone sends a message to [email protected], the sending server checks the MX records for example.com. The DNS lookup then returns the mail server, or servers, that handle incoming email for that domain.

Think of an MX record as an address book for email delivery. The recipient’s domain tells the sending server which organization it needs to reach, while the MX record tells it which mail server to contact.

Step 4: The Email is Relayed to the Recipient’s Mail Server

Once the sending server knows where the message needs to go, it connects to the recipient’s mail server using SMTP. The two servers communicate with each other and go through the steps needed to transfer the email. If the recipient’s server accepts the message, that part of the relay has been completed successfully.

The email does not always travel directly from the sender’s server to the final receiving server. Depending on the setup, it may pass through an additional relay, security gateway, or third-party email service first. For example, a business may send its emails through a third-party SMTP email relay before those messages reach Gmail, Microsoft, Yahoo, or another email provider.

A simple version of the journey looks like this:

Sender → Sending SMTP server → SMTP relay → Recipient’s mail server → Recipient’s inbox

The exact path can be different from one email setup to another. Some messages may only involve a sending server and a receiving server, while others may pass through multiple systems before reaching their destination.

Step 5: The Recipient’s Mail Server Checks the Email

Reaching the recipient’s mail server does not automatically mean the email will land in the inbox. The receiving server first needs to decide whether it trusts the message. It can check several technical and reputation-based signals to determine whether the email looks legitimate, suspicious, or potentially harmful.

Here are the basic checks:

• SPF, which verifies whether the server sending the email is authorized to send messages for the sender’s domain.
• DKIM, which verifies the digital signature attached to the email to verify that it was signed by an authorized domain and has not been altered in transit.
• DMARC, which evaluates whether SPF or DKIM passes with the required domain alignment and tells the receiving server what to do when authentication fails.
• IP reputation, which looks at the history and reputation of the IP address sending the message.
• Domain reputation, which looks at signals associated with the sending domain and its past email activity.
• Spam signals, which check for patterns that are commonly associated with spam or unwanted messages.
• Message content, which scans the email, links, and attachments for suspicious or potentially harmful content.

These checks explain why successful SMTP relay does not always mean successful inbox delivery. The sending server may successfully hand off the email, but the recipient’s mail provider can still reject it, quarantine it, or place it in the spam folder.

Step 6: The Recipient’s Mail Server Accepts or Rejects the Message

Once the recipient’s mail server finishes checking the email, it decides whether to accept or reject it. If the email passes the required checks, the server accepts it and moves it forward for delivery. If something looks wrong, the server can reject the message and send an SMTP response that explains the reason.

For example, the email may be rejected because the recipient’s address does not exist, the sender’s IP has a bad reputation, email authentication has failed, or the message does not follow the server’s security rules. The SMTP response code helps the sending server understand what went wrong.

However, a failed delivery does not always mean the email is permanently rejected. Sometimes the problem is temporary, such as the recipient’s mail server being down or too busy to accept new messages. In this case, the sending server can try to deliver the email again later. If the problem is permanent, the sender usually needs to fix the issue before the email can be delivered.

Step 7: The Email Reaches the Recipient’s Mailbox

If the recipient’s mail server accepts the email, it puts the message into the recipient’s mailbox. The recipient can then open it using Gmail, Outlook, Apple Mail, webmail, or another email app.

This is the point where the main SMTP delivery process is complete. The email has traveled from the sender’s system, through the required mail servers and relays, and finally reached the server that handles the recipient’s email.

One important thing to remember is that SMTP is mainly used to send and transfer emails. It is not normally used to read emails from your mailbox. Protocols such as IMAP and POP3 are used to retrieve messages, while services such as Gmail and Outlook give you an interface to read and manage them.

Simply put, SMTP moves the email from one mail system to another, while the recipient’s mail system takes care of storing and showing the message.

SMTP Relay Example

A real-world example makes this whole process easier to picture. Imagine you run an online store and a customer places an order. As soon as the payment goes through, your website needs to send an order confirmation to the customer’s email address.

The website could run its own mail server, but that would mean dealing with email infrastructure, delivery, authentication, reputation, security, and other technical details. Instead, the website can connect to an SMTP email relay service and let that service handle the email delivery.

The basic flow would look like this:

Online store → SMTP relay service → Customer’s mail server → Customer’s inbox

Here’s what happens behind the scenes:

• The customer places an order: The website creates an order confirmation using the customer’s email address and order details.

• The website connects to the relay: The application sends the message to its configured SMTP relay using the provider’s SMTP credentials or API.

• The relay accepts the message: The relay checks the sending request and ensures the application is authorized to use the service.

• The relay finds the destination: It looks up the MX records for the customer’s email domain to find the right receiving mail server.

• The relay sends the email: It connects to the customer’s mail server and transfers the message using SMTP.

• The receiving server checks it: The customer’s email provider checks SPF, DKIM, DMARC, IP reputation, domain reputation, spam signals, and other security rules.

• The message is delivered: If the receiving server accepts the email, it is placed in the customer’s mailbox.

This is one of the main reasons businesses use SMTP relay services. The online store does not have to build and maintain an entire email delivery system just to send order confirmations. Instead, it can focus on its application while the relay service handles the SMTP delivery infrastructure.

Why Do Businesses Use SMTP Relay?

Email is a big part of how many businesses operate every day, and an SMTP relay provides a simple way to send emails. Here are some common reasons businesses use SMTP relay:

Better Email Delivery Management

An SMTP relay gives a business one place to manage its outgoing emails. Instead of every website, app, or server sending emails separately, they can all send their messages through the same relay. This makes things easier to manage. If emails suddenly stop being delivered, the team has one main system to check instead of searching through several different servers and applications.

Easier Scaling

The number of emails a business sends can grow very quickly. A small website might only send a few hundred emails a day, while a larger company may need to send thousands or even millions. An SMTP relay service is built to handle large amounts of email. So, as the business grows, it can send more messages without having to keep building and upgrading its own email server.

Better Monitoring

When an email leaves your website or application, you still need to know what happens to it. Many SMTP relay services provide logs and reports that show whether an email was delivered, delayed, bounced, or rejected. This can make email problems much easier to find. 

For example, if hundreds of emails suddenly start bouncing, the business can check the relay’s reports and start looking for the cause.

Improved Security

An email server needs to be protected from people who may try to misuse it. If an SMTP server is not set up properly, attackers could use it to send spam or other unwanted emails.

SMTP relay services can provide security features such as authentication, TLS encryption, rate limits, and access controls. These features help make sure that only approved users and applications can send emails through the relay.

Separation of Email Traffic

Businesses often send different types of emails, and it can be useful to keep them separate. For example, employee emails do not have to use the same system as automated emails from a website or marketing emails.

A company might use:

• Employee email: Microsoft 365 or Google Workspace
• Transactional email: SMTP relay service
• Marketing email: Email marketing platform

Keeping these emails separate makes them easier to manage and track. It can also make it easier to find delivery problems because the business can see which type of email is causing the issue.

More Control Over Automated Emails

A lot of business emails are sent automatically. For example, a website may send an email when someone creates an account, resets a password, places an order, or receives an important alert.

An SMTP relay gives these websites and applications a reliable way to send those messages. Businesses can also control which applications are allowed to use the relay, helping prevent unauthorized systems from sending emails.

The Future of SMTP Relay and Email Delivery

As businesses send more emails through websites, apps, and other online tools, they need a reliable way to deliver all those messages. SMTP relay can make this easier by handling email delivery without requiring businesses to build everything on their own. As email systems grow, businesses will also need to focus more on security, authentication, and making sure emails reach the right place. A properly set up SMTP relay can help with all of this. Understanding how SMTP relay works now can help businesses build an email system that is ready to grow with them.